The present invention relates to a reciprocating internal combustion engine, and specifically to a reciprocating engine employing a rockable cam capable of oscillating within limits so as to directly push a valve lifter of an intake valve.
A well-known direct-driven valve operating mechanism that a valve lifter of an engine valve is driven or pushed directly by means of a cam (hereinafter is referred to as xe2x80x9cfixed camxe2x80x9d) formed as an integral section of a camshaft, is superior to a rocker-arm type or a lever type, in compactness, design simplicity, and enhanced rotational-speed limits. In the direct-driven valve operating mechanism, in order to provide a wide range of contact between the cam surface of the fixed cam and the valve lifter without undesirably eccentric contact in a very limited contact zone, generally the axis (the center of rotation) of the camshaft lies on the prolongation of the centerline of the valve stem of the engine valve (each of intake and exhaust valves). Thus, the center distance between the center of the intake-valve camshaft and the center of the exhaust-valve camshaft is in proportion to the angle between the center of the intake-valve stem and the center of the exhaust-valve stem. As is generally known, in typical reciprocating internal combustion engines, a crankpin is connected to a piston pin by means of a single link known as a xe2x80x9cconnecting rodxe2x80x9d. In such single-link type reciprocating engines, for the purpose of reduced side thrust acting on the piston, the crankshaft axis (crankshaft centerline) lies on the cylinder centerline, as viewed from the axial direction of the crankshaft. The assignee of the present invention has proposed and developed a variable valve operating mechanism (see FIG. 4) continuously varying a valve lift characteristic (at least a valve lift and a working angle) and widely applied to the previously-discussed direct-driven valve gear layout. In the variable valve operating mechanism as shown in FIG. 4, in order to drive an intake-valve operating mechanism, a drive shaft is laid out parallel to the crankshaft axis, in a similar manner as the typical camshaft having fixed cams formed as integral sections of the camshaft. A rockable cam is rotatably fitted onto the outer periphery of the drive shaft such that the oscillating motion of the rockable cam is permitted within predetermined limits and the valve lifter is pushed directly by the cam surface of the rockable cam. Changing an initial phase of the rockable cam continuously changes the valve lift characteristic. For instance, when the rockable cam is used in the intake-valve operating system instead of using the fixed cam, it is desirable that the center of oscillating motion of the rockable cam (that is, the axis of the drive shaft) is offset from the centerline of the valve stem of the intake valve, from the viewpoint of a widened contact area between the cam surface of the rockable cam and the valve lifter and reduced side thrust acting on the valve lifter associated with the intake valve. However, if only the drive shaft of the intake valve is simply offset from the center of the intake-valve stem, the geometry and dimensions between the intake-valve drive shaft and the crankshaft become different from the geometry and dimensions between the exhaust-valve camshaft (or the exhaust-valve drive shaft) and the crankshaft. In such a case, the engine design including a power transmission system layout from the crankshaft to the drive shaft (or the camshaft) has to be largely changed. The assignee of the present invention has also proposed and developed a multi-link type reciprocating engine employing a variable piston stroke characteristic mechanism (see FIG. 2) continuously varying a compression ratio. In case of such multi-link type reciprocating engines, taking account of the magnitude of load applied to each link as well as piston side thrust, it is undesirable to arrange the crankshaft centerline on the cylinder centerline viewed from the axial direction of the crankshaft. However, the simple offset of only the drive shaft of the intake valve from the center of the intake-valve stem, leads to the problem of the differences between (i) the geometry and dimensions between the intake-valve drive shaft and the crankshaft and (ii) the geometry and dimensions between the exhaust-valve camshaft (or the exhaust-valve drive shaft) and the crankshaft.
Accordingly, it is an object of the invention to provide a reciprocating internal combustion engine employing a rockable cam capable of oscillating within predetermined limits so as to directly push a valve lifter of an intake valve, which avoids the aforementioned disadvantages.
It is another object of the invention to provide an improved layout among a cylinder centerline, a crankshaft centerline, a center of oscillating motion of a rockable cam (i.e., a center of an intake-valve drive shaft), and a center of an intake-valve stem, in a reciprocating internal combustion engine employing the rockable cam capable of oscillating within predetermined limits so as to directly push a valve lifter of the intake valve.
In order to accomplish the aforementioned and other objects of the present invention, a reciprocating internal combustion engine comprises a cylinder block having a cylinder, a piston movable through a stroke in the cylinder, an intake valve, an intake-valve lifter on a stem of the intake valve, an intake-valve drive shaft that rotates about its axis in synchronism with rotation of a crankshaft, a rockable cam that is rotatably fitted on an outer periphery of the intake-valve drive shaft, and that oscillates within predetermined limits during rotation of the intake-valve drive shaft so as to directly push the intake-valve lifter, and as viewed from an axial direction of the crankshaft, an axis of the intake-valve drive shaft being offset from a centerline of the intake-valve stem in a first direction that is normal to both a centerline of the cylinder and an axis of the crankshaft and directed from the cylinder centerline to an intake valve side, and the crankshaft axis being offset from the cylinder centerline in the first direction.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.